The present invention relates to a method and apparatus for evaluating the characteristics of an insulating film provided on a semiconductor substrate, particularly an ultra-thin gate insulating film.
As the degree of integration in a semiconductor integrated circuit device has increased significantly in recent years, elements including transistors in a MIS semiconductor device have been required to be smaller in size and higher in performance to correspond to the higher degree of integration. As the elements including the transistors have become smaller in size and higher in performance, the implementation of a MIS (Metal Insulator Semiconductor) structure with high reliability has been in greater demand. To provide a MIS structure with higher reliability, each of the components of the MIS structure which are a gate electrode (Metal), a gate insulating film (Insulator), and a semiconductor substrate (Semiconductor) should have high reliability.
The gate insulating film which is one of the elements composing the MIS structure is expected to be rapidly reduced in thickness to correspond to further miniaturization and higher-speed and lower-voltage operation of the transistors. It is expected that an extremely thin insulating film with a thickness of 2 nm or less will be used actually in the 21st century. The implementation of an excellent gate insulating film is so important an issue as to say that the characteristics of the gate insulating film determine the characteristics of the MIS transistor and therefore the electric characteristic of the semiconductor integrated circuit device.
As a material composing a gate insulating film, a silicon dioxide (SiO2) has been used conventionally. However, the limited TDDB (Time Dependent Dielectric Breakdown) reliability of a gate oxide film, i.e., SiO2 film is expected to be among factors which impair the reliability of an LSI device in the future. If the thickness of the gate oxide film is reduced increasingly to reach 2 nm or less, the problem arises that a tunneling current induced by direct tunneling of carriers through the gate oxide film, i.e., a gate leakage current is further increased. In a system LSI, in particular, such an increased leakage current incurs a significant increase in the power consumption of the LSI device so that a large number of new materials replacing SiO2 have been proposed as materials composing the gate insulating film in terms of power consumption (Reference: 1999-ITRS Roadmap). Thus, the increased leakage current in the gate oxide film has requested revolutions in the development of a gate insulating film and new materials as well as in quality control at a manufacturing site.
As a test for examining the reliability of a gate insulating film, a so-called TDDB test has been performed conventionally in an accelerated environment. The TDDB test performed under the accelerated environment is a method for examining the lifetime of an insulating film till it reaches a dielectric breakdown by applying a voltage higher than a working voltage, increasing the temperature, examining the current-voltage characteristic (I-V characteristic), and judging that the insulating film breaks down when a current increases abruptly. In the TDDB test, it is general to examine the I-V characteristic and determine, as the lifetime, the time elapsed till an abrupt change is observed in leakage current, while monitoring an amount of leakage current. During the test, measurement using a MIS structure which is a capacitor occupying a large area has been performed normally in terms of examining a defect density in the gate insulating film.
For in-line evaluation, a so-called Hg prober test has also been used widely in which the I-V characteristic and the like are. evaluated by pressing a mercury terminal functioning as a gate electrode against a gate insulating film that has been formed previously to omit the step of forming the gate electrode composing a MIS structure (see, e.g., Japanese Unexamined Patent Publication No. HEI 06-140478). The Hg prober method is performed primarily for the development of an insulating film material, control in the process of fabricating an insulating film, a reliability test for an insulating film, or the like.
In general, the breakdown of a gate insulating film in each of the foregoing tests is judged by an abrupt increase in leakage current. In the TDDB test, a MIS structure (which is also a MIS capacitor) having a gate area of 0.01 mm2 or more is used widely to provide a sufficient sensitivity with which a current value is detected. In the evaluation method using a Hg prober also, the contact area between the mercury terminal and the gate insulating film is as large as 0.01 mm2 or more due to the structure of the mercury terminal.
As a result of a further increase in leakage current induced by a recent reduction in the thickness of the gate insulating film, it may be difficult to judge the time at which the gate insulating film breaks down in a test for evaluating the quality of a gate insulating film by observing the I-V characteristic as described above. In addition, a phenomenon termed a false breakdown in a thin film on a 2-nm level, which is different from the conventional breakdown, has made the judgment of a breakdown difficult. A description will be given herein below to the cause of the difficulty by using the TDDB test as an example.
FIG. 1 is a view showing the I-V characteristic of a thermal oxide film (SiO2 film) with a thickness of 1.5 nm provided on a p-type silicon substrate by using a gate area as a parameter. In the drawing, the horizontal axis represents a gate voltage (V) and a vertical axis represents the absolute value of a gate leakage current (A) As can be seen from the drawing, the gate leakage current value increases as the gate area increases to reach 3 xcexcm2, 30 xcexcm2, and 300 xcexcm2. If a comparison is made between leakage current densities obtained by dividing the gate leakage values by the respective gate areas, however, it will be understood that each of the leakage current densities has a nearly equal value. In FIG. 1, there are also shown the I-V characteristics of gate insulating films with respective thicknesses of 1.5 nm and 2.5 nm that have broken down. It will be understood that a substantially uniform characteristic is obtained after the dielectric breakdown irrespective of a difference in film thickness whether the thickness of a gate insulating film is 2.5 nm or 1.5 nm. This may be because the dielectric breakdown has occurred at a certain local leakage spot in the gate insulating film.
However, as the gate area becomes larger to reach 3 xcexcm2, 30 xcexcm2, and 300 xcexcm2, the timing T with which the gate leakage current increases abruptly (the time of breakdown) for the judgment of breakdown becomes less distinct, as shown in FIG. 1. When the gate insulating film has a large thickness, the timing with which the gate leakage current increases abruptly is distinct as indicated by the broken curve in the drawing. However, the distance between the initial I-V characteristic curve and the post-breakdown I-V characteristic curve is reduced as the thickness of the gate insulating film is reduced, so that it becomes more difficult to recognize the time T at which the gate insulating film breaks down.
FIG. 2 is a view showing the relationship between the gate area and the gate leakage current by using a gate voltage as a parameter, which is for determining the critical value of the gate area over which the breakdown of the gate insulating film cannot be detected any more. In the drawing, the horizontal axis represents the gate area (xcexcm2) and the vertical axis represents the gate leakage current (A). By way of example, FIG. 2 shows the case where the thicknesses of the gate insulating films are 1.5 nm and 2.5 nm. In the I-V characteristics shown in FIG. 1, the gate leakage current Ig is determined uniquely if the gate voltage Vg and the gate area Sg are determined. It may be said that the relationship shown in FIG. 2 is obtained by plotting the values of the leakage current Ig at a given gate voltage Vg in FIG. 1.
If the gate leakage current is Ig, the gate current density is Dg, and the gate area is Sg, the following expression:
Ig=Dgxc3x97Sg
is satisfied so that the following expression:
log Ig=log Dg+log Sg
is also satisfied. Since the gate leakage current Ig is necessarily represented by a straight line having a gradient of 1 relative to the gate area Sg in the coordinate system shown in FIG. 2, if a gate leakage current Ig when a given gate voltage Vg is applied to a gate insulating film having a gate area Sg is determined in FIG. 1, the Ig-Sg characteristic curve of the gate insulating film having a given thickness is determined by drawing a straight line having a gradient of 1 and passing through a point on the coordinates determined by the gate area Sg and the gate leakage current Ig. On the other hand, the gate leakage current Ig after the breakdown of the gate insulating film has a constant value determined by the gate voltage Vg irrelevantly to the gate area Sg so that the post-breakdown Ig-Sg characteristic curve is represented by a straight line parallel to the horizontal axis. It is to be noted that, in FIG. 2, the segments of the pre-breakdown Ig-Sg characteristic lines located above the points of intersection with the post-breakdown Ig-Sg characteristic lines are obtained by simply extending the segments thereof located below the points of intersection and therefore do not actually exist.
At each of the points of intersection between the pre- and post-breakdown characteristic lines, the gate leakage current Ig before the breakdown of the gate insulating film coincides with the gate leakage current Ig after the breakdown. Since the pre- and post-breakdown I-V characteristic lines thus coincide with each other in the I-V characteristic of the gate insulating film having the gate area Sg at the point of intersection shown in FIG. 1, the time at which the gate leakage current Ig changes abruptly is barely observed and the time of breakdown cannot be detected.
In other words, for a gate insulating film having a given thickness to which a given gate voltage Vg is applied, it is difficult to detect the time of breakdown unless a gate area Sg which causes a gate leakage current Ig smaller than the gate leakage current Ig at the point of intersection between the pre-breakdown Ig-Dg characteristic curve and the post-breakdown Ig-Sg characteristic curve by a specified margin is provided. If it is assumed that the thickness of the gate insulating film is 1.5 nm and the gate voltage Vg in use is xe2x88x923 V, it is necessary to perform measurement with a gate area which causes a gate leakage current Ig smaller, by a specified margin, than the gate leakage current Ig at the point of intersection (the point at which the gate area corresponds to about 1000 xcexcm2) between the pre- and post-breakdown Ig-Sg characteristic curves of the gate insulating film with a thickness of 1.5 nm shown in FIG. 2, e.g., with a gate area of 20 xcexcm2 or less for ensured detection of the time of the breakdown of the gate insulating film.
Such a disadvantageous phenomenon is also observed during evaluation performed by using the Hg prober so that it is structurally difficult to reduce the transverse cross section of the mercury terminal functioning as the gate electrode of the MIS structure to a size proper to evaluate a gate insulating film on a 1.5-nm level.
It is therefore a primary object of the present invention to provide a method and apparatus for evaluating an insulating film which ensures, even if the insulating film such as a gate insulating film has been reduced in thickness, the detection of the time at which the insulating film breaks down by causing a measuring terminal for evaluating the insulating film for the characteristics or thickness thereof to function as the gate electrode of a MIS structure.
A method for evaluating an insulating film of the present invention is a method for evaluating an insulating film provided on a conductor layer in a substrate for characteristics or dimensions thereof, the method comprising the steps of: (a) disposing a measuring member having a plurality of conductor bumps and wires connected to the conductor bumps above the substrate, while placing the conductor bumps and the insulating film in opposing relation; (b) bringing the conductor bumps into contact with the insulating film and pressing the conductor bumps and the insulating film against each other with a given pressing force; and (c) applying an electric stress between the conductor bumps and the conductor layer and thereby evaluating the insulating film for the characteristics or dimensions thereof.
When the conductor bumps of the measuring member are pressed against the insulating film with the specified pressing force in accordance with the method, a nearly uniform contact area is provided between each of the conductor bumps and the insulating film. The use of the conductor bumps easily reduces the contact area between each of the conductor bumps and the insulating film to about 200 xcexcm2 or less, which is different from the conventional TDDB test performed by using a HG prober and a large-area MIS capacitor. As a result, the evaluation of a gate leakage current (I-V characteristic) in a gate insulating film reduced in thickness to, e.g., about 1.5 nm can be performed with high accuracy.
The step (c) can include evaluating the insulating film for a leakage characteristic thereof, for reliability thereof under the electric stress, for a current-voltage characteristic thereof, for a dielectric constant thereof, or for a thickness thereof.
The step (b) can include reducing a pressure in a space between the substrate and the measuring member and thereby relatively pressing the conductor bumps and the insulating film against each other. Even if a large number of conductor bumps are used, the method allows a nearly equal pressing force to be applied to each of the conductor bumps over the entire substrate.
The step (b) may include controlling the pressing force with which the conductor bumps and the insulating film are pressed against each other such that a contact area between each of the conductor bumps and the insulating film falls within a specified range, thereby allowing more accurate evaluation. The pressing force can also be controlled with a relative distance between the measuring member and the substrate.
The step (c) includes evaluating the insulating film, while heating at least one of the substrate and the measuring member, thereby adjusting the contact area resulting from the deformation of each of the conductor bumps to a proper value or allows an acceleration test to be performed on the characteristics of the insulating film.
The method for evaluating an insulating film of the present invention further comprises, prior to the step (a), the step of: (d) calibrating the pressing force to keep a contact area between each of the conductor bumps and the insulating film within a specified range, thereby increasing the reliability of evaluation.
The pressing force can be calibrated by using a second substrate having a second insulating film on a second conductor layer and bringing the conductor bumps of the measuring member into contact with the second insulating film or by evaluating the second insulating film for characteristics thereof including a leakage current.
The method for evaluating an insulating film of the present invention further comprises, after the step (c), the step of: (e) bringing the conductor bumps and the insulating film into a non-contact state and relatively moving the measuring member and the substrate, the procedure from the step (e) to the step (d) being repeated a plurality of times. The arrangement increases evaluation accuracy by performing evaluation at a large number of portions, while suppressing size variations by reducing the number of the conductor bumps.
The method for evaluating an insulating film of the present invention further comprises, prior to the step (a), the step of: (f) preparing a database individually storing respective sizes of the conductor bumps, wherein the step (c) includes retrieving, from the database, data on the respective sizes of the conductor bumps and evaluating the insulating film for the characteristics or dimensions thereof. The arrangement increases evaluation accuracy.
The method for evaluating an insulating film of the present invention further comprises, prior to the step (a), the step of: (g) predicting deformation of each of the conductor bumps in the step (b) based on data on respective sizes of the conductor bumps and preparing a database individually storing a contact area between each of the conductor bumps and the insulating film resulting from the deformation of each of the conductor bumps, wherein the step (c) includes retrieving, from the database, data on the contact area between each of the conductor bumps and the insulating film and evaluating the insulating film for the characteristics or dimensions thereof based on the contact area of each of the conductor bumps. The arrangement increases evaluation accuracy.
The step (g) preferably includes applying a pressing force between the measuring member and the substrate such that at least some of the plurality of conductor bumps are plastically deformed, measuring an area of an upper surface of each of the conductor bumps that have been plastically deformed after removing the pressing force, and predicting the deformation of each of the conductor bumps in the step (b) based on the area.
A first apparatus for evaluating an insulating film of the present invention is an apparatus for evaluating an insulating film provided on a conductor layer in a substrate for characteristics or dimensions thereof, the apparatus comprising: a measuring member having one or more conductor bumps and a wire connected to each of the conductor bumps, the measuring member being smaller in size than the substrate; and pressing-force adjusting means for adjusting a relative pressing force exerted on each of the conductor bumps and the insulating film such that the relative pressing force falls within a specified range.
The arrangement allows for evaluation to be performed only with respect to the insulating film on a part of the substrate so that the contact area between each of the conductor bumps and the insulating film is adjusted easily to be uniform and high evaluation accuracy is provided.
The apparatus for evaluating an insulating film of the present invention further comprises moving means for laterally moving the substrate or the measuring member, thereby allowing the insulating film on the entire substrate to be evaluated for the characteristics or dimensions thereof, while providing a uniform contact area between each of the conductor bumps and the insulating film.
The first apparatus for evaluating an insulating film of the present invention further comprises moving means for relatively rotating the substrate or the measuring member, thereby increasing evaluation accuracy by increasing the number of portions to be evaluated, while reducing variations in the sizes of the conductor bumps by reducing the number of conductor bumps.
By providing the pressing-force adjusting means with a mechanism for reducing a pressure in a space between the substrate and the measuring member, an equal relative pressing force is exerted on each of the conductor bumps and the insulating film uniform over the entire substrate.
Preferably, each of the conductor bumps is composed of a material having a lower hardness than the insulating film.
Preferably, a size of each of the conductor bumps is set such that a contact area between each of the conductor bumps and the insulating film resulting from the pressing force falls within a specified range.
The first apparatus for evaluating an insulating film of the present invention further comprises: a memory unit for individually storing a size of each of the conductor bumps; and an arithmetic operational unit for performing an arithmetic operation with respect to the characteristics or dimensions of the insulating film based on a size of each of the conductor bumps, which increases the evaluation accuracy of the apparatus for evaluating an insulating film.
A second apparatus for evaluating an insulating film of the present invention is an apparatus for evaluating an insulating film provided on a conductor layer in a substrate for characteristics thereof, the apparatus comprising: a measuring member having one or more conductor bumps, a wire connected to each of the conductor bumps, and a dummy bump which is not used to evaluate the insulating film for the characteristics thereof; and pressing-force adjusting means for adjusting a relative pressing force exerted on each of the conductor bumps and the insulating film such that the relative pressing force falls within a specified range.
In the arrangement, the pressing force applied to the measuring member is received by the conductor bumps and the dummy bumps so that variations in the pressing force applied to each of the conductor bumps are suppressed.
A third apparatus for evaluating an insulating film of the present invention is an apparatus for evaluating an insulating film provided on a semiconductor substrate for characteristics thereof, the apparatus comprising: a measuring member having at least one cantilever fixed at a proximal end thereof to have one or more conductor bumps mounted on a lower surface of a tip portion thereof and a wire connected to each of the conductor bumps; and pressing-force adjusting means for adjusting a relative pressing force exerted on each of the conductor bumps and the insulating film such that the relative pressing force falls within a specified range by controlling an amount of displacement of the cantilever.
The arrangement allows control of the relative vertical positional relationship between the bump support member and the substrate and allows proper adjustment of the relative pressing force which defines the contact area between each of the conductor bumps and the insulating film.
A fourth apparatus for evaluating an insulating film of the present invention is an apparatus for evaluating an insulating film provided on a conductor layer in a substrate for characteristics thereof, the apparatus comprising: a measuring member having one or more conductor bumps and a wire connected to each of the conductor bumps; a rod-like member supported rotatably at a proximal end thereof; amount-of-movement detecting means for detecting an amount of movement of a tip of the rod-like member along a surface of the insulating film; and pressing-force adjusting means for adjusting a pressing force exerted on each of the conductor bumps and the insulating film such that the pressing force falls within a specified range depending on a value detected by the amount-of-movement detecting means.
The arrangement allows control of the relative vertical positional relationship between the bump support member and the substrate and allows proper adjustment of the relative pressing force which defines the contact area between each of the conductor bumps and the insulating film.
The amount-of-movement detecting means optically detects the amount of movement of the tip of the rod-like member, which allows the pressing force to be controlled with a constantly high accuracy with a simple structure.
A fifth apparatus for evaluating an insulating film of the present invention is an apparatus for evaluating an insulating film provided on a semiconductor substrate for characteristics thereof, the apparatus comprising: a measuring member having one or more conductor bumps and a wire connected to each of the conductor bumps; pressing-force adjusting means for adjusting a relative pressing force exerted on each of the conductor bumps and the insulating film such that the relative pressing force falls within a specified range; and a substrate stage comprising a conductor projection for braking a portion of the insulating film on a back surface of the semiconductor substrate to come in contact with the back surface of the semiconductor substrate.
In the arrangement, the conductor projection breaks the portion of the insulating film on the back surface of the semiconductor substrate to come into contact with the back surface thereof so that substrate contact for transmitting an electric signal is obtained reliably without providing an extra step.
Preferably, the conductor projection is composed of a material containing at least one of rhenium, rhodium, nickel, tungsten, and tantalum.